1. Field of the Invention
The invention relates to a system for speech enhancement in a room comprising a microphone arrangement for capturing audio signals from a speaker's voice, means for processing the captured audio signals and a loudspeaker arrangement located in the room for generating amplified sound according to the processed audio signals.
2. Description of Related Art
By a system of the above-mentioned type, a speaker's voice can be amplified in order to increase speech intelligibility for persons present in the room, such as the listeners of an audience or pupils/students in a class room. Such speech enhancement systems often encounter feedback problems, especially when used with lapel microphones (when the speaker is moving around in the room, feedback conditions are always changing, the minimum stable gain must be selected leading to poor intelligibility; on the other, hand feedback cancellers reduce the intelligibility when in feedback condition). Feedback problems are less severe when boom microphones (which need less gain since they are located very close to the speaker's mouth) are used; however, most speakers prefer to use lapel microphones rather than boom microphones.
An example of a speech enhancement system is described in International Patent Application Publication WO 2010/000878 A2 and corresponding U.S. Patent Application Publication 2012/221329, wherein the audio signal processing includes a feedback canceller which analyzes the captured audio signals in order to determine whether there is a critical feedback level caused by feedback of sound from the loudspeaker arrangement to the microphone arrangement (Larsen effect). The feedback canceller outputs a status signal indicating the presence or absence of feedback conditions to a main control unit in order to reduce the system gain when feedback conditions occur.
German Patent Application DE 25 26 034 A1 and corresponding U.S. Pat. No. 3,894,195 relate to a hearing aid wherein the microphone signals, after having passed an automatic gain control (AGC) stage, undergo frequency shifting by 10 Hz in order to reduce feedback, so that the maximum gain can be increased by about 10 dB.
U.S. Pat. No. 5,394,475 relates to audio systems providing for a frequency shift of the audio signals in order to reduce feedback, wherein it is mentioned that the frequency shift may be about 5 Hz.
U.S. Pat. No. 4,237,339 relates to the use of directional microphones for feedback reduction in an audio teleconferencing system, wherein the loudspeaker and the microphones are rigidly mounted on a boom and the microphones are located and oriented relative to the loudspeaker in such a manner that the null position of the directivity is directed towards the loudspeaker.
European Patent Application EP 0 581 261 A1 relates to the use of a Wiener filter for feedback reduction in a hearing aid, wherein the Wiener filter is implemented as part of a filter controlled by a user operated control. JP 2008-141734 A and corresponding U.S. Pat. No. 8,311,234 relate to the use of a Wiener filter for feedback reduction in a hands-free telephone system or a video conference system. EP 1 429 315 A1 and corresponding U.S. Pat. No. 7,068,798 relate to the use of a Wiener filter for feedback reduction in a vehicle communication system.